For a multi-layer wiring substrate on which semiconductor elements are mounted, a multi-layer wiring substrate in which a buildup layer is formed on both surfaces of a core substrate is widely used. Here, the core substrate is a substrate to be a support substrate of the buildup layer, and includes an insulating layer and a wiring pattern on it, and as the insulating layer, generally, a rather rigid layer in which a thermosetting resin such as an epoxy resin is impregnated in a reinforcing material such as glass cloth and cured is used. Also, the buildup layer is a layer that is layered on the wiring pattern of the core substrate and includes an insulating layer and a wiring pattern on it. In recent years, together with thinning of the multi-layer wiring substrate and density increase of the wiring pattern, a multi-layer wiring substrate not including a core substrate has been provided. The multi-layer wiring substrate not including the core substrate (sometimes called a “coreless substrate”, hereinafter) has merits that it is easy to be thinned and the wiring pattern can be formed in a high density.
As a method for fabricating such a coreless substrate, a method for fabricating a multi-layer wiring substrate by forming a laminate having a desired number of insulating layers and a wiring pattern on both surfaces of a metal foil clad laminate to be a support substrate, then separating the laminate from the support substrate, and performing subsequent processes to the separated laminate has been devised (see Patent Literatures 1 to 4). Also, a method for fabricating a multi-layer wiring substrate using a support substrate (sometimes called a “dummy core”) which is used only in a fabricating process and does not configure the multi-layer wiring substrate itself to be a product in this way is called a coreless method hereinafter.
In the coreless method like this, as a method for forming a non-through hole for an interlayer connection of a multi-layer wiring substrate, a method (conformal method) for providing an opening on metal foil at a laser processing scheduled position, emitting a laser beam aiming at the opening, and processing an insulating layer thereunder with the metal foil as a mask is generally used.